In the Third Generation Partnership Project (the 3GPP project) for cellular wireless systems, a mechanism referred to as Discontinuous Reception (DRX) will be introduced. One purpose of DRX is to save battery time in user terminals.
By means of the DRX mechanism, a user terminal will be able to turn on and off radio resources for a certain amount of time, based on configured parameters and specified rules.
As an example of a DRX mechanism, mention might be made of the so called Continuous Packet Connectivity mechanism, CPC, for WCDMA systems, in which a DRX scheme is specified. According to this scheme, a user terminal initiates continuous usage of its radio resources (continuous reception) as soon as it receives data during a non-DRX period, and resumes a DRX state based on a “timeout” following a period in time during which no data is received.
In 3G (third generation) systems, as in many other wireless cellular systems, there is a controlling node, in 3G referred to as eNodeB, which has as one of its purposes to control traffic to and from user terminals within a certain area, a cell, in the system. In order for a DRX mechanism to function properly, a set of clear rules are needed to enable the eNodeBs of the system to determine, at all times, the state of “their” user terminals with respect to DRX, i.e. DRX or not.
In 3G systems, the solutions currently envisioned for DRX schemes don't properly take into account the fact that different kinds of transmissions or retransmissions may be used between the eNodeBs and their respective UEs. Examples of such different kinds of (re-)transmissions are transmissions which are caused by the use of HARQ, Hybrid Automated Repeated Request, and those which are caused by the use of Radio Link Control, RLC.
Current solutions for DRX schemes assume either:                that only one transmission is needed, or        that if one or more retransmissions and/or if multiple RLC segments are needed, those are handled by the DRX scheme as first transmissions, or        that once data is received, the UE will listen for an “on duration” period of time, regardless of whether or not retransmissions or new data is expected of not, which means that the radio resources will always be committed for a longer period than the actual need for listening, as the UE goes on listening until it assumes the DRX state, which is done after the expiration of a configured inactivity timer.        
Generally, a packet can be transmitted using dynamic scheduling, by means of which explicit signalling is used by the eNodeB to notify the UE that data is coming, or using semi-persistent scheduling, in which the first transmission from the eNodeB is detected by the UE using so called “blind detection”, and retransmissions from the eNodeB are made using scheduling assignments, as with dynamic scheduling.
In HARQ transmissions, for downlink transmissions, the UE sends a HARQ NACK to its eNodeB to request a retransmission. The eNodeB does not have advance knowledge that a retransmission will be needed, and can thus not signal in advance using MAC signalling to order the UE to assume a shorter DRX period to allow for retransmissions. A retransmission can occur after a certain amount of time, typically measured in milliseconds, during which time no resources will be used for this transmission.
In addition, with semi-persistent scheduling, which is likely to be used for VoIP, retransmissions are made using scheduling assignments just as with dynamic scheduling. The general problem that the retransmission will be delayed by the length of the DRX period is thus independent of the type of service or transmission.